1. Field of the Invention
This invention relates to positive photoresist compositions and a process for producing the same, as well as alkali-soluble novolak resins that can advantageously be used in such compositions. More particularly, the invention relates to positive photoresist compositions that have high feature integrity in spite of high sensitivity and which yet provide a great depth of focus. The invention also relates to a process for producing such positive photoresist compositions and alkali-soluble novolak resins that can advantageously be used in such compositions. The invention further relates to a method of forming patterns with high reproducibility using the stated compositions.
2. Description of Related Art
In the process of fabricating semiconductor devices such as ICs and LSIs and liquid-crystal devices such as LCDs, positive photoresist compositions comprising the combination of film-forming alkali-soluble resins and a photosensitive component made of quinonediazido group containing compounds are used commercially on account of their high resolution, sensitivity and resistance to etching.
With the ever increasing degree of integration in semiconductor devices and liquid-crystal devices, a demand has arisen in VLSI production to process superfine patterns in as high precision as to realize feature sizes in a submicron range or not more than half a micron meter. Under the circumstances, the positive photoresist composition to be used must satisfy various requirements in characteristics such as the highest possible feature integrity without impairing high sensitivity, and a great enough depth of focus to reproduce resist patterns faithful to a mask pattern without being affected by undesired phenomena such as standing waves and random reflection from a substrate having steps on the surface.
The previous attempts at improving these characteristics have largely been directed to phenols in alkali-soluble resins, the photosensitive component made of quinonediazido group containing compounds and a sensitizer component made of poly-phenolic compounds.
The relationship between sensitivity and feature integrity is generally one of trade-off and an attempt at increasing sensitivity results in a lower feature integrity due to pronounced narrowing of the unexposed areas and vice versa.
Take, for example, the case of improvement of phenols, in which the major problem has been with the relative proportions of different kinds of phenols. A photoresist containing a resin that has a large proportion of m-cresol having comparatively high reactivity features high sensitivity but is poor in resolution due to a small depth of focus. If the proportion of p-cresol which is less reactive than m-cresol is increased, the depth of focus increases to provide better resolution but, on the other hand, the sensitivity decreases. Under the circumstances, a method has been attempted in such a way that the proportion of p-cresol is increased and the conditions for polymerizing the resin component is varied as appropriate to make an oligomer (a polymer of low molecular weight), thereby ensuring that there will be no drop in the sensitivity of the resulting photoresist. However, this approach has not yet succeeded in attaining sufficient characteristics to be fully compatible with today's microfabrication technology.
Another technique that has been proposed to date comprises producing an alkali-soluble resin by forming a polymer using a mixture of formaldehyde and ketone or an aldehyde of two or more carbon atoms as an agent for polycondensing phenols (see Unexamined Published Japanese Patent Application No. 204146/1993). However, even in this case, no characteristics have yet been attained that are sufficient to be fully compatible with today's microfabrication technology.
The present inventors conducted intensive studies with a view to solving the above-mentioned problems of the prior art. They prepared a polycondensation reaction product (alkali-soluble novolak resin) using a phenolic compound and a mixed aldehyde as a polycondensing agent that was a combination of an unsaturated aliphatic aldehyde and a saturated aliphatic aldehyde in specified amounts and used this resin in a positive photoresist composition. The resulting positive photoresist composition retained high sensitivity and yet had high feature integrity while providing a great depth of focus. The present invention has been accomplished on the basis of this finding.